1. Field of the Invention
This invention relates to ultraviolet (UV) or electron beam (EB) curable compositions. More particularly this invention relates to UV/EB curable cationic inks and coatings for use in ink jet, gravure and flexographic printing.
2. Description of Related Art
Cationic inks and coatings are well known for very good adhesion on different plastic and metallized substrates. Conventional cationic, ultraviolet or electron beam (hereinafter identified as UV/EB) curable inks and coatings typically are comprised of cycloaliphatic epoxy components which upon exposure to light of suitable wavelengths and in presence of suitable cationic photoinitiators, polymerizes to form a hard film with desired properties. One limitation of cycloaliphatic epoxides is their sensitivity to moisture which slows the cure rate. W. Watt reviewed the UV curing of epoxides in The UV Curing of Epoxides, Chemistry & Technology of UV Curing, Vol. 2, pp. 247-82, 1980. W. Watt notes in paragraph 5.4 that water will increase termination of cationic polymerization, and that high humidity has been shown to greatly increases the time for UV-curable epoxide coatings to become tack-free. In a more recent publication by R. F. Eaton et al. entitled Formulating Concepts For Both High Solids And Water Based UV Cured Cycloaliphatic Epoxy Wood Coatings, Rad. Tech. North America Proceedings, Vol. 1, pp. 384 et seq., 1990, they disclose that cycloaliphatic epoxies can be readily dispersed in water to form UV curable zero VOC emulsions which have coating performance equivalent to solvent applied cationic UV coatings. However, Eaton et al. also note that the rate of cure is significantly slowed by high humidity even though adding water to formulations does not slow cure.
Curable cationic coating compositions containing epoxy (i.e., oxirane) compounds and oxetane compounds are disclosed in Igarashi et al., U.S. Pat. No. 5,674,922; Takami et al., U.S. Pat. No. 5,721,020; and Kansai Paint Co., UK Patent Application GB 2310211. Igarashi et al. discloses active energy beam-curable compositions containing a compound with one oxetane ring and hydroxyl group in the molecule; a compound with one oxirane ring in the molecule; and a compound that initiates cationic polymerization. Igarashi's compositions are solvent free and viscosity is adjusted by varying the component ratios. Takami et al., discloses an ultraviolet-curing coating composition for cans which contains a cation-polymerizable compound, a compound having at least one oxetane ring in the molecule, a cationic initiator, and a lubricity imparting agent, i.e., a wax. Takami et al. also discloses that the coating composition can be applied by a variety of coating methods and that the resulting coating is heated to remove solvents if present. In the examples, solventless liquid compositions containing diepoxides and 3-ethyl-3-hydroxymethyloxetane were coated on a variety of substrates for testing. Kansai Paint Co. discloses an active energy ray-curable resin composition which contains a resin having an oxetane functional group as well as an epoxy group in the molecule; and a cationic polymerization initiator. Kansai Paint Co. discloses that their composition can be prepared by mixing the components in the presence of a solvent or a dispersing agent. Specific solvents disclosed are hydrocarbons, halogenated hydrocarbons; alcohols, ethers, ketones, and esters. Kansai Paint Co. further discloses that their formulation may further contain a monomeric oxetane such as 3-ethyl-3-hydroxymethyloxetane.
Although advances have been made in radiation curable cationic polymerization coating compositions, the curing of these coating compositions remains sensitive to humidity and require long cure times. In addition, viscosity adjustment of these compositions is frequently limited, particularly for solventless systems, thus limiting their use in applications such as low viscosity inks. Further, the coating components employed, particularly solvents, frequently emit vapors and/or odors which are undesirable for environmental or safety reasons. Accordingly, there is a need for materials used in fast curing cationic inks and coatings which are insensitive to humidity, have low viscosity, and use low odor diluents. These needs are met by the composition of the present invention.